According to the development of integrated circuits, the line-width of ultra-large-scale integrated circuit is smaller than one quarter micron meters. A traditional photolithography process is performed for to the formation of quarter-micron-meters lines in integrated circuits but a single line is narrower than one quarter micron meters. In the traditional deep ultraviolet (DUV) photolithography process, an isolated-line pattern has a smaller depth of focus (DOF) than that a dense-line pattern has. Some approaches are tried to solve the above issue for the isolated-line pattern.
A next-generation DUV stepper machine uses a kind of light, which has a wavelength about 0.193 nanometers, to expose photoresist. At the recent years, the DUV stepper machines, which are popular used in factories, use light that has wavelength about 0.248 nanometers. Using the next-generation stepper machine, the DOF of the isolated line pattern must be increased. Nevertheless, the new stepper machine can not develop ultra-violet photoresist pattern very well.
Other approach to increase the DOF of the integrated circuits is to use a phase-shifting mask during a photolithography process. A phase-shifting mask (PSM) is consisted of several masking layers and attenuated layers. The phase-shifting mask has a special structure in order to generate an electrical field, which has a good contrast, being applied on the mask when the light of a stepper machine transmits the mask. However, the cost for manufacturing a phase-shifting mask is so expensive. Thus, the phase-shifting mask is not popularly adapted in wafer factories. Additionally, sub-resolution assisted features are placed around an isolated line on a mask so that the DOF of the isolated line could be increased. Nevertheless, to fabricate the assisted features on a mask needs complex calculations and designs. So, this approach is not available for semiconductor's factories.
A surface pretreatment is used to harden the surface of a DUV photoresist layer. Conventionally, the surface pretreatment "is performed by using DI water or developer" and it is named as a liquid-phase pretreatment. In a liquid-phase pretreatment, the photoresist layer is dipped in DI water or developer before the developing process of the photoresist layer. But, the liquid-phase pretreatment is rough to the DUV photoresist layer. The profile of the DUV photoresist for defining an isolated line, which is exposed, has a "T-shape" to "cross section after the liquid-phase pretreatment". As the isolated-line DUV photoresist layer has a T-shape cross section, the critical dimension of the DUV photoresist can not be definitely determined. Moreover, the liquid-phase pretreatment is not uniform enough. Referring to FIG. 1, a cross section's view of a photoresist layer 10 not being treated is shown. In spite of the photoresist layer 10 has a sharp profile but the top surface of the photoresist layer 10 are round and the critical dimension of the isolated line under the photoresist layer 10 will be hard to control during following etching processes. Referring to FIG. 2, a cross sectional view of a DUV photoresist layer 10, which is treated by using a liquid-phase pretreatment and is developed, is shown and it has a T-shape cross section.
According to the above discussion, a new surface pretreatment for the DUV photoresist is needed to increase the DOF of an isolated-line pattern.